The nation's major automakers told Congress yesterday the government must ease existing fuel-economy standards to eliminate present "front-loading" of mileage-increase requirements, or risk putting the industry in a costly bind.

Testifying before a House Commerce subcommittee, a panel of industry executives contended that meeting the current standards would require an investment of about $80 billion -- double what automakers have spent over the past eight years.

They also asserted that "stretching out" the mileage-increase requirements, to provide for more even year-to-year increases between now and 1985, would make only a marginal difference in gasoline savings.

Meanwhile, industry sources said General Motors Corp. will ask federal regulators to allow automakers to meet government emissions-control standards on the basis of fleetwide averages rather than for each car individually.

The shift, if approved by the government, would place emissions standards compliance on the same basis as fuel-economy standards. Industry analysts insisted it would keep overall standards intact but give automakers more flexibility.

The testimony on the gasoline-mileage standards marked the first public salvo in an effort by Detroit to prod the National Highway Traffic Safety Administration into softening the present requirements by administrative action.

Joan Claybrook, the agency's director, said earlier this week officials would complete a preliminary assessment of the industry's case in a month. But, she cautioned, "I'm not inclined to go change -- every time someone sneezes."

The subcommittee's hearings this week are intended primarily to give the issue an airing. Congressional sources say the panel has no formal legislation drafted and is not planning any at this time.

Under existing regulations, the fuel economy standard is scheduled to rise from 19 miles a gallon in the current model year to 20 in 1980. It then would increase by two miles a gallon each year to 27.5 mpg in 1985.

The industry wants to "smooth" this out to provide for a "straight-line" 1.5-mile-a-gallon increase each year initially, and possibly also to reduce the 1985 target to 26 miles a gallon rather than the present 27.5.

It also wants the government to allow automakers who exceed those standards in any one year to claim the overage as a credit any time in the following three years. Current law provides for only a one-year carry-forward.

The bid for a shift to "straight-line" increases was endorsed yesterday by the liberal United Auto Workers union, which argued it would be better to "give a little" on an economic issue than compromise on emissions standards.

However, they were opposed by Clarence M. Ditlow III, director of the Ralph Nader-affiliated Center for Auto Safety, who asserted the "front-end-loading" would prove important in the face of projected gasoline shortages.